LayerRendererChromium is a compositor thread-side class, so Canvas(2D)LayerChromium should not have access to it, except during updateCompositorResources. The DrawingBuffer creation and destruction and its interaction with the LRC are probably the largest problems here. The removal and addition of child contexts will have to be deferred until the next commit and stored somewhere (CCLayerTreeHost?).
Also, maybe setDrawingBuffer could be removed, as it doesn't seem to be used anywhere except to clear the drawing buffer in some other class's destructor.
I don't think we actually need the child context stuff any more, that was for some latching code that's redundant now. I'll confirm that with Bates.
I think we're good here now, DrawingBuffer only touches the canvas context (not the compositor context) and there's no child context stuff any more. Marking as fixed.